1. Field of the Invention
This invention relates to a method of sealing open ends of channels of a ceramic honeycomb structural body which is used for purifying harmful gases such as automobile exhaust gas or industry-plant exhaust gas, and more particularly the invenion relates to a method of sealing open ends of channels of a ceramic honeycomb structural body which forms a heat exchanger or a ceramic filter for removing particulates from automobile exhaust gas or a ceramic honeycomb structural body for supporting catalysts.
2. Description of the Prior Art
Recently, ceramic honeycomb structural bodies have been used as catalyst carriers for purifying automobile exhaust gas or as heat exchangers, and much attention has been paid to various advantages of such ceramic honeycomb structural bodies; namely, that pressure loss of gas flow therethrough is low because a large number of substantially straight and parallel channels are uniformly distributed in the ceramic honeycomb structural body, that the surface area per unit volume is large, and that the ceramic honeycomb structural body can be heated up to a high temperature by a small amount of heat energy because the channels thereof are surrounded only by thin walls.
For instance, it is known to make a ceramic honeycomb filter by taking advantage of the fact that the ceramic honeycomb structural body has a large surface area per unit volume and that the partition walls or inside walls of the ceramic honeycomb structural body have a large number of channels therein and the channels are porous and very thin. More specifically, certain channels of the ceramic honeycomb structural body are sealed by a suitable sealing material at one end thereof, while the remaining channels of the ceramic honeycomb structural body are sealed by a suitable sealing material at an opposite end thereof, so that the partition walls form filtering layers for providing a large filtering area per unit volume of the ceramic honeycomb filter while ensuring a low pressure loss therethrough. In this filter, the thin porous partition walls capture particulates from the exhaust gas passing therethrough, so as to perform an effective filtration. Accordingly, the sealing material which seals the end openings of the channels of such ceramic honeycomb filter is required to be tightly bonded to the peripheral walls and partition walls so as to form a perfect seal without any leakage of the dust-containing gas. Thus, reliable sealing of the open ends of the ceramic honeycomb structural body channels is the most important point in producing the ceramic honeycomb filters. When the ceramic honeycomb structural body is used as a carrier of catalysts, open ends of the channels in the proximity of the outer peripheral wall of the honeycomb structural body are sometimes sealed by a suitable sealing material for the purpose of improving the mechanical strength thereof, and the sealing material of such a catalyst-supporting ceramic honeycomb structural body is also required to be tightly bonded to the partition walls of the honeycomb structural body.
As to the production of such ceramic honeycomb filters, the following methods are known: namely, a method disclosed by Japanese Patent Application Laid-open Specification No. 7215/1982 which comprises steps of adhering a film onto at least one end surface of a ceramic honeycomb structural body, boring holes on the film at positions corresponding to desired open ends of the channels to be sealed, and pressing a dilatant sealing material into the desired open ends of the channels; a method similar to the last mentioned method except that viscous sealing material is introduced into the desired open ends of channels to be sealed by dipping; and a method comprising steps of adhering a film onto at least one end surface of a ceramic honeycomb structural body, boring holes on the film at positions corresponding to those open ends of the channels which are to be left open, introducing epoxy resin into the open ends of the channels to be left open by dipping in a manner similar to that of the last mentioned methods, hardening the epoxy resin thus introduced, removing the film, and introducing a sealing material under pressure into the desired open ends to be sealed.
The above-mentioned conventional method using the pressing of the dilatant sealing material into the desired open ends of the channels has a shortcoming in that, when the film is bored at the positions corresponding to the desired channels open ends to be sealed, each bored hole of the film must be exactly the same in shape and size as that of the corresponding channels open end to be sealed, and if the hole of the film is too small, gap are likely to be formed between the sealing material and the partition walls of the honeycomb structural body channels so as to cause serious defects in the performance thereof. Besides, even if special needle-like jigs are used, it is difficult to perforate the film at a reasonably high speed while ensuring exactly the same size and shape of the film holes as those of the honeycomb structural body channels to be sealed without chipping the porous fragile sidewalls of the honeycomb structural body channels. It is also difficult to produce satisfactory sealing of irregularly-shaped channels or cells of the honeycomb structural body at the periphery thereof. The sealing material used in this method is required to be dilatant, and when the major ingredients of the sealing material are plastic raw materials such as clay and minerals, it is very complicated to prepare suitable binders of the sealing material because such ingredients tend to render a large thixotropy to the sealing material body.
In the above-mentioned method using the introduction of the viscous sealing material by dipping, difficulties have been experienced in that the sealed end surfaces tend to be uneven, and that, when the honeycomb structural body has a low cell density or when the channels of the honeycomb structural body have a large cross-sectional area, the dipping operations must be repeated a number of times.
The above-mentioned method using the introduction and hardening of epoxy resin in the channels to be left open has shortcomings in that the process of the method is time-consuming, and that the thermal expansion of epoxy resin is, in general, larger than that of the ceramic material of the honeycomb structural body and tends to crack the honeycomb structural body, so that extra care must be paid to selection of suitable epoxy resin.